1. Field of the Invention
The present invention relates to a method and apparatus for controlling a thermal printer head for use with a thermo-autochrome recording medium.
2. Description of the Related Art
Fuji Photo Film Corporation has developed a technology called "Thermo-Autochrome" (TA). The TA technology is based on a recording medium of a structure as shown in FIGS. 7A.about.7E. The recording medium includes a support 101 (e.g., a sheet of paper), a cyan-forming layer 102 coated on the support 101, a magenta-forming layer 103 coated on the cyan-forming layer 102 and a yellow-forming layer 104 coated on the magenta-forming layer 103.
The cyan-forming layer 102, the magenta-forming layer 103 and the yellow-forming layer 104 include similar coloring mechanisms and, hence, only the coloring mechanism of the yellow-forming layer 104 is described in detail as an example. Although not shown, it should be understood that the yellow-forming layer 104 includes a yellow-forming diazonium salt compound contained in capsules and a coupler surrounding the capsules. The yellow-forming diazonium salt compound and the coupler are both colorless when not combined with each other. When the yellow-forming layer 104 is heated to a temperature range, the coupler migrates into the capsules. Thus, the yellow-forming diazonium salt compound and the coupler are combined so as to become a dye of yellow. After the dye of yellow is formed, the yellow-forming layer 104 is irradiated by ultra-violet (UV) light with a wavelength range so as to decompose the yellow-forming diazonium salt compound which has not been combined with the coupler, thus fixing the color of yellow.
Color discrimination is achieved by designing the cyan-forming layer 102, the magenta-forming layer 103 and the yellow-forming layer 104 so as to react in different temperature ranges. Accordingly, a full color image is formed in the following steps sequentially:
1. the yellow-forming layer 104 is heated by a heating unit 105 to a first temperature range and is then irradiated by UV light 106 with a first wavelength range, thus forming and fixing the color of yellow 108; PA1 2. the magenta-forming layer 103 is heated by the heating unit 105 to a second temperature range higher than the first temperature range and is then irradiated by UV light 107 with a second wavelength range smaller than the first wavelength range, thus forming and fixing the color of magenta 109; and PA1 3. the cyan-forming layer 102 is heated by the heating unit 105 to a third temperature range higher than the second temperature, thus forming the color of cyan 110; it is to be understood that after the formation of the color of cyan 110, the cyan-forming layer 102 is not irradiated by UV light, because it normally will not be subject to a temperature sufficient high to cause the coupler to migrate into the capsules.
Briefly speaking, each of the color-forming layers has both functions of "color formation by heat" and "color fixing by UV light".
In recording, it is difficult therefore cost-inefficient to monitor the temperature of the TA recording medium to precisely control the color density of the TA recording medium. That is, the data regarding the relation between the color density and the temperature is not practically useful in precisely providing desired color density. Hence, instead of using the diagram depicting the relation between the color density of the TA recording medium and the temperature of the TA recording medium, a plurality of diagrams similar to FIG. 8 are used. FIG. 8 shows a relation between the color density of the TA recording medium and the heat provided to the TA recording medium at a certain ambient temperature. The ambient temperature is measured in an appropriate position within a thermal printer and is issued to be the temperature of the TA recording medium. The curves as shown in FIG. 8 should be moved to the right if the ambient temperature is lowered and should be move to the left if the ambient temperature is increased.
In a conventional thermal printer utilizing the TA technology, in developing a full color image, a fixed set of data representing a relation between the color density of the TA recording medium and the heat provided to the TA recording medium at an ambient temperature is used. A drawback of the conventional thermal printer will be explained referring to FIG. 9. At an ambient temperature T1, an amount of heat En must be provided to the TA recording medium to provide a desired color density D. However, in developing a full color image, the ambient temperature may be increased to T2. In this case, a color density D' will be obtained instead of the desired color density D if the amount of heat En is provided to the TA recording medium.
In addition, since heat is provided to the recording medium by a heating unit of the thermal printer head, as seen in FIG. 7A.about.7E, if a thermal printer head is not completely cooled down after applying heat to the recording medium, residual heat may accumulate on the thermal printer head, thus changing the amount of heat provided to the recording medium and affecting the printed color density.
Therefore, some methods and circuits are provided to eliminate the aforementioned drawback of thermal printing. As disclosed in U.S. Pat. No. 4,797,837, thermoelectric heat pumps are utilized to cool the thermal printer head. A sensed thermal printer head temperature is digitized and is compared with a reference temperature for determining whether operation of the heat pumps should be initiated or halted. However, introduction of thermoelectric heat pumps raises the cost and complicates the control mechanism. Furthermore, it is the thermal printer head that is cooled by the heat pump, not the recording medium, variation of printed color density on the recording medium caused by temperature change is not reduced effectively.
Another method for historical control of thermal printing is disclosed in U.S. Pat. No. 5,377,159. A historical control technique controls the drive current fed to a heating unit according to the printing or non-printing of the last few dots by that heating unit, thereby overcomes problems caused by residual heat accumulated on the heating units. This prior art solves the problem of residual heat on a heating unit caused by printing of previous dots; however, variation of printed color density caused by environmental temperature change is not taken into account.